PURPOSE: Children and adolescents are encouraged to maintain a habitually active lifestyle because of the known health benefits associated with regular physical activity, but there are some reports that a high level of activity may be associated with increased fracture risk. This prospective controlled exercise intervention study in prepubertal children evaluated if a school-based exercise intervention could enhance growth-related gains in muscle strength and muscular function without affecting fracture risk. METHODS: Fractures were registered in 417 girls and 500 boys age 7-9 yr in the intervention and in 836 age-matched girls and 872 boys. The intervention included 40 min.d of school physical education for 2 yr, whereas the controls achieved 60 min.wk. In a subsample consisting of 49 girls and 80 boys in the intervention and 50 girls and 53 boys in the control group, body composition was measured by dual-energy x-ray absorptiometry, muscle strength by isokinetic peak torque (PT) of the knee extensors, and flexors at 60 and 180 degrees .s by a computerized dynamometer and neuromuscular performance by vertical jump height. RESULTS: The rate ratio (95% confidence interval) for children in the intervention group to sustain a fracture was 1.07 (0.66-1.68). The annual gain in knee extensor PT at 180 degrees .s was significantly higher for both girls (P < 0.001) and boys (P < 0.01) in the intervention compared with the control group. Boys in the intervention group also had a greater annual gain in knee flexion PT at 180 degrees .s (P < 0.001), and girls had a greater gain in vertical jump height (P < 0.05). CONCLUSION: An increase in school-based physical education from 60 to 200 min.wk enhanced muscle strength in prepubertal children without affecting fracture risk.

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